1. Field of the Invention
This invention relates to a differential master cylinder adapted for use as fluid pressure generating source in a brake system.
2. Description of the Prior Art
Popularization of use of disc brakes for motor vehicles has necessitated use of a fluid pressure cylinder arrangement of relatively large piston diameter in general for obtaining a large braking force by pressing friction pads against a rotor. This necessity causes an increase in the quantity of brake fluid required to be transferred in the initial stage of a braking operation for covering a gap existing between each friction pad and the rotor before the pad comes to abut upon the rotor. Accordingly, a master cylinder which generates braking fluid pressure is generally arranged to be of large diameter. Besides, to obtain high braking fluid pressure from the use of a master cylinder of large diameter, there is provided a force multiplying device for a pedalling force in combination with a brake pedal in most cases.
Further, to prevent any energy loss by preventing the dragging of the friction pads by the rotor during a non-brake applying condition in accordance with a recent tendency of energy saving, it is necessary to retain a required extent of gap between the rotor and the friction pad. This, therefore, also calls for use of a master cylinder of still larger diameter and a larger force multiplying device.
Meanwhile, there have been various efforts to obviate the disadvantage of increased weight, etc. resulting from the above stated tendency of increasing the size of a brake system. As a result of such efforts, there have been provided arrangements called differential master cylinders. Each of these differential master cylinders is provided with larger and smaller cylinder portions of different diameters. These larger and smaller cylinder portions are arranged to perform two functions separately from each other. One is to transfer a large quantity of the fluid at the initial stage of brake application while almost no fluid pressure is required. The other is to produce a high degree of fluid pressure required for obtaining a large pressing force after the friction pads have abutted on the rotor.
Generally, such conventional differential master cylinders are of the type wherein a fluid passage provided for the purpose of replenishing the smaller cylinder portion with the fluid from the larger cylinder portion is merely arranged to be opened and closed by means of a piston which slidably engages both the larger cylinder portion and the smaller cylinder portion. However, since the fluid passage is of a relatively narrow structure, the conventional differential master cylinders have a shortcoming in respect to the stability of their operations at the time of quick or sudden application of pressure (or sudden brake application). Further, since a mechanism for opening and closing the fluid passage is arranged within the piston, the arrangement complicates the construction of the conventional differential master cylinder. Therefore, it has been hardly possible to obtain a sufficiently great force multiplying effect.